where is states that demonstrable consent is not needed?
Nowhere.
Before the UETAs and ESIGN, electronic documents had no legal standing. Even if you and the customer agreed to use e-documents, state and federal courts could (and probably did) ignore them. The state UETAs and ESIGN change all of that.
UETAs tell the state judges that e-documents are legal--provided the sender and recipient agree to use them. No demonstration is required--just consent. State laws govern contracts...but not federal disclosures (TIS, TIL, etc.)
ESIGN tells state and federal judges that e-documents are legal, but it imposes the additional burden of the demonstration
IF the recipient is a consumer. It says nothing about other types of recipients, so e-deliveries to businesses would only require a simple agreement (more or less like EUTA.) Since ESIGN is federal law, demonstrable consent (Section 7001) makes your federal e-disclosures (TIS, TIL, etc.) the legal equivalent of paper.